Wireless adhoc networks consist of independent wireless nodes that dynamically form connections with each other to create a network. An adhoc network does not require, although it can benefit from, any central node or centralized infrastructure or traditional cellular network. Instead, the ad-hoc network can grow or shrink “on its own” by means of state of the art discovery mechanisms and individual node decisions.
In order for the wireless nodes to form adhoc networks, there is a need for a standardized set of protocols that allows nodes to communicate and specifically for an authentication procedure that facilitates establishing secure communication links within the nodes of the adhoc network.
For example, Bluetooth adhoc networks may use the Secure Simple Pairing (SSP) protocol to exchange messages that are necessary for device discovery, connection establishment and exchanging public security keys. In addition, SSP allows for the so called I/O capability exchange that contains rudimentary information about the physical communication capabilities of the nodes.
It is expected that future wireless ad-hoc nodes will be able to provide a much broader range of services to one another than what adhoc networks are typically used for today. For instance, certain services may require that the connection is retained over a longer time. Some of these services may also be characterized by frequent transmission interruption. Another challenge would be that some services such as gaming require faster access after an interruption. Maintaining a continuous connection over a longer duration especially when transmission or reception is intertwined by inactive periods is power inefficient. Thus, efficient power saving techniques in adhoc networks are particularly beneficial and desirable when a variety of services are introduced in the near future.
Some sort of power saving mechanism is employed in all traditional cellular networks. In particular, the power saving or the so-called discontinuous reception (DRX) especially in idle mode is used in all cellular systems. In more advanced technologies such as long term evolution (LTE) or in later releases of UTRAN (UMTS Terrestrial Radio Access Network), the DRX operation is also used in the connected or active mode.
The DRX operation in idle mode and in connected mode allows the user equipment (UE) to save battery power while it enables it to camp on a cell for receiving paging and to stay connected for receiving user specific data or control information respectively. In both idle mode and in connected mode, depending upon the DRX cycle in use, the UE can stay in DRX mode for unlimited amount of time. However, in connected mode, depending upon the network implementation, the UE typically goes to idle state after a long period of inactivity, which is generally controlled by the network.
Another type of power saving technique is discontinuous transmission (DTX) at the UE. Although the original intention of the DTX operation is to reduce the received interference at the base station, lower transmission activity also enables the UE to save its battery.
The introduction of the DTX operation was particularly advantageous in traditional systems like WCDMA, where regardless of the data transmission activity, a bidirectional continuous control channel is maintained primarily to enable a closed loop power control operation. However a continuous or at least quasi-continuous control channel may also be employed in other systems such as LTE to retain synchronization, fast scheduling etc. Thus, in principle, the concept of DTX can be used in a wide range of technologies and scenarios.
In typical network implementations both DRX and DTX operations run in parallel, thus saving both battery power and interference.
Hitherto the power saving techniques such as DRX or DTX have been primarily developed for the UE. However, due to high energy costs and in order to reduce the effect of global warming, there is considerable interest in developing energy efficient wireless network nodes such as energy efficient base stations or techniques which enable the nodes to operate at lower power. Thus, the future base stations may also operate in sleep mode. Advanced methods are being developed to ensure that lower base station transmission activity does not significantly hamper the normal UE operation.
In existing ad hoc networks no explicit DRX mechanism exists. The lack of DRX operation has a number of implications:
Firstly, for time critical services e.g. voice over IP (VOIP), due to the lack of viable DRX mechanism, a typical ad hoc UE receiver has to stay continuously active. This obviously drains UE battery power. For the human end user this implies that the active talk time would be drastically different depending on cellular or ad-hoc modes, which would probably be unacceptable for most users.
In case of non time critical services e.g. interactive services, an ad hoc UE typically releases connection after an inactivity period. This is because from a UE power consumption point of view it is not feasible to stay active continuously after a certain level of inactivity. However, interactive sessions generally last longer but are characterized by sporadic transmissions followed by long inactivity periods.
Hence, without active mode DRX, the UE is forced to loose connection and go into idle mode. This means that even if a higher layer level session is not completed, the UE has to undergo a complete access procedure to re-access the resources for transmissions after each inactivity period. In general, this increases the delay and also increases the risk of failure. In ad hoc networks, the connection re-establishment may take even longer than in usual classical networks. Certain interactive services such as gaming require relatively faster reaction time. Thus, without active mode DRX, it is challenging to support a wide range of services in ad hoc networks, unless the UE battery power consumption is compromised.